Contents
- How to Convert Inches to mm
- Formula
- Inches to mm Conversion Table
- Real-World Examples
- For Students
- Frequently Asked Questions
- Related Conversions
How to Convert Inches to mm
1 inch = 25.4 mm. This is an exact definition, not a rounded value. Half an inch is 12.7 mm, and 10 inches equal 254 mm.
Inches and millimeters are both common length units, but they belong to different measurement systems. The inch is part of the imperial system used mainly in the United States, while the millimeter belongs to the metric system used nearly everywhere else. You will run into this conversion when working with hardware specs, screen sizes, pipe fittings, or any product labeled in imperial units.
- Enter your inch value in the calculator above.
- The calculator multiplies your value by 25.4 to get the result in millimeters. To do it by hand, multiply the same way: inches x 25.4 = mm.
- For example, 3 inches x 25.4 = 76.2 mm.
Formula
mm = inches x 25.4
Multiply the number of inches by 25.4 to get millimeters. The factor 25.4 is exact because one inch was officially defined as exactly 25.4 millimeters in 1959 by international agreement.
Worked example: 8 inches x 25.4 = 203.2 mm. So an 8-inch tablet screen measures 203.2 mm diagonally.
Inches to mm Conversion Table
| Inches | mm |
|---|---|
| 0.25 | 6.35 |
| 0.5 | 12.7 |
| 0.75 | 19.05 |
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 12 | 304.8 |
| 15 | 381 |
| 20 | 508 |
| 25 | 635 |
| 36 | 914.4 |
| 48 | 1,219.2 |
| 50 | 1,270 |
| 72 | 1,828.8 |
| 100 | 2,540 |
Real-World Examples
A standard smartphone screen measures about 6.1 inches diagonally, which equals 154.94 mm. Phone case manufacturers and screen protector makers list specs in both units, so knowing the conversion helps when ordering accessories.
Lumber in the US is sold in imperial sizes. A “2×4” board is actually 1.5 inches by 3.5 inches, meaning the true dimensions are 38.1 mm by 88.9 mm. Metric countries label these same boards differently, so the conversion matters when following building plans from another country.
Pipe fittings often come in inch-based sizes. A common 3/4-inch pipe (0.75 inches) has an outer diameter of about 19.05 mm. Plumbers switching between imperial and metric fittings use this conversion daily.
TV and monitor sizes are measured in inches worldwide. A 27-inch monitor has a diagonal of 685.8 mm. Knowing this helps when checking whether a monitor will fit a specific desk mount or wall bracket rated in millimeters.
For Students
The inches-to-millimeters conversion appears in science and shop classes, SAT prep, and engineering coursework. The key fact to remember: 1 inch = exactly 25.4 mm. This is not an approximation. A quick memory trick: 25.4 has three digits, and “inch” also has a short, punchy feel, so pair them together as a fixed fact.
A common mistake is confusing millimeters with centimeters. There are 10 mm in 1 cm, so 1 inch = 25.4 mm = 2.54 cm. Make sure you check which metric unit the problem asks for before multiplying. Try converting 7.5 inches and 16 inches to mm using the calculator above to build confidence with the formula.
Frequently Asked Questions
How many mm are in one inch?
There are exactly 25.4 millimeters in one inch. This is a defined value, not a measured approximation. It has been the international standard since 1959.
Is an inch bigger than a millimeter?
Yes, one inch is much larger than one millimeter. A single inch contains 25.4 mm, so it takes about 25 millimeters lined up end to end to equal one inch.
How do I convert inches to mm without a calculator?
Multiply the inch value by 25.4. For a rough estimate, you can multiply by 25 and add half of the original value. For example, 4 inches: 4 x 25 = 100, plus 4 x 0.4 = 1.6, giving 101.6 mm.
Why do some countries still use inches?
The United States, Liberia, and Myanmar are the only countries that have not fully adopted the metric system. In the US, inches remain standard for construction, manufacturing, and everyday measurements. Many industries worldwide still use inches for legacy compatibility, especially in aviation and screen sizing.
What is the difference between mm and cm?
A millimeter (mm) is one-tenth of a centimeter (cm). There are 10 mm in 1 cm. When converting from inches, multiply by 25.4 to get mm, or by 2.54 to get cm. Use mm when you need finer precision, such as in engineering or machining.